"never ever respond" implies that you already know they're a stalker -- but by that point you've surely already had some correspondence with them, unless you never reply to anyone. So how do you tell when it actually crosses the line?

Good point. If a stalker starts out as 'nice' (and boy, can they be nice) like in the story above then you must feel that there is a point where they go from normal to 'creepy' (hard to define). If you've been in regular communication with them until that point then it will be impossible to avoid some of the fall-out, but the rules still hold. It's a war of attrition, and unless your local law enforcement takes this sort of thing serious (and oh, the joys of the internet, you can now stalk someone half the planet away with proxies, anonymous remailers and so on to hide who you are) it can go on for years.

Not all stalkers are created equal. Some will fold after the slightest hint of official repercussions, others will continue no matter what.

Whatever happens it'll leave you changed in how you behave online and in real life.